1. Field of the Invention
The present invention relates to the method for desulfurizing steel which uses premelted synthetic slags and more particularly uses premelted synthetic slag for desulfurizing molten steel which synthetic slag is obtained as a co-product from vanadium and ferrovanadium processing.
2. Description of the Prior Art
Molten steel ladle metallurgical practices employ synthetic slag for desulfurization for a number of reasons. Synthetic slags provide a thermal insulation for the molten metal top surface and protect the molten metal from atmospheric oxidation. The synthetic slag ladle processing requires little additional equipment or additional capital costs. Synthetic slag ladle processing practices can achieve desulfurization as low as 0.005% (wt.) residual sulfur content and can remove 50% and more of the molten metal sulfur content. Such snythetic slags heretofore have had a high calcium oxide content and have included aluminum oxide, and occasionally calcium fluoride (as a flux), silicon dioxide and metallic aluminum or aluminum alloys. Improvements in the synthetic slag ladle processing of ;molten steel results when the synthetic slag is premelted to reduce the porosity and tendency to absorb moisture prior to use. Typically equal weights of calcium oxide and aluminum oxide are mixed and premelted together to form calcium aluminate. The premelted calcium aluminate is added to the ladle, either by itself, or with lime, fluorspar (to supply calcium fluoride) and occasionally some metallic aluminum.
Note that the premelted synthetic slag should be distinguished from prefused synthetic slag. The distinction is that prefused slag is created by a solid state reaction occurring at a temperature below the melting temperature of the slag. The resulting prefused synthetic slag tends to be porous and nonuniform. The premelted synthetic slag is obtained by actually melting the synthetic slag ingredients above their melting temperature to establish a liquid mixture which is allowed to cool to form the premelted synthetic slag.
To reduce the temperature required for melting synthetic slags, calcium fluoride as a fluxing agent frequently is added to the synthetic slag mixture. The presence of fluoride in the resulting slag tends to increase the wear on the refractory linings of molten steel ladles which are typically fabricated from cast dolomitic lime and/or high alumina ceramics and/or high MgO ceramics. Existing ladle practices encourage loss of magnesium oxide from the ladle lining into the ladle slag. Said loss requires frequent ladle lining replacement. The MgO loss can be detected by the increase of MgO content of the slag. Any process which lowers loss of MgO from the ladle lining will reduce the number of relinings and reduce the downtime needed to replace ladle linings.